Alkyl or alkaryl hydroxamic acids and their salts are well-known collectors for the froth flotation of oxide minerals. Soviet workers have found a variety of applications for such alkyl hydroxamic acids, such as those described by Pradip and Fuerstenau, Mineral Flotation with Hydroxamate Collectors, Reagents in the Minerals Industry, Ed. M. J. Jones and R. Oblatt, Inst. Min. Met., London, 1984, pp. 161-168, a recent review that summarizes the flotation application of alkyl hydroxamic acids.
Hydroxamic acids have been used for the flotation of metals or minerals such as pyrochlore, fluorite, huebnerite, wolframite, cassiterite, muscovite, phosphorite, hematite, pyrolusite, rhodonite, chrysocolla, malachite, barite, calcite, and rare-earths. They are generally more powerful and more selective then conventional fatty acids, fatty amines, petroleum sulfonates and alkyl sulfates. However, the commercially employed methods of making alkyl or alkaryl hydroxamic acid or its salts are tedious and unsafe from the point of view of industrial production.
A procedure for making potassium alkyl hydroxamate is disclosed in Organic Synthesis, Vol. II, page 67. In the disclosed process, solutions of KOH and NH.sub.2 OH--HCl in methanol are combined. After the KCl byproduct is filtered off, the filtrate is combined with a liquid mixture of methyl caprylate and methyl caprate, and, after standing for 24 hours, the product crystals are filtered off. Major drawbacks of this method include low yields, the use of a large amount of toxic and flammable methanol, and the use of potassium hydroxide, which is more expensive than sodium hydroxide. In addition, the industrial scale filtration of a methanolic reaction mixture is clearly undesirable from a safety standpoint.
U.S. Pat. No. 3,933,872 to Hartlage claims an improved method of making fatty hydroxamates. Hydroxylamine sulfate and the methyl ester of a fatty acid are reacted in the presence of dimethylamine in an anhydrous lower alcohol slurry. The free hydroxamic acids formed are neutralized with dimethylamine or an alkali metal base to yield an ammonium or alkali metal salt, which precipitates, and is filtered and dried. However, the disclosed procedure also employs flammable lower alcohols, such as methanol, ethanol or isopropanol, requiring the filtration of the final hydroxamic product, which is hazardous. Moreover, because of the heterogeneous nature of the reaction, the reaction rate is very slow, e.g., on the order 15 hours in methanol and 5 days in isopropyl alcohol, and the yields are relatively low, i.e., on the order of about 75 percent.
Various Russian workers have reported methods for making alkyl hydroxamic acids and their salts in aqueous alkaline media. Gorlovski, et al., Vses. Soveshch. po Sintetich. Zhirozamenitelyam, Poverkhnostnoaktivn, Veschestvam i Moyushchim Sredstvam, 3rd, Sb., Shebekino, 1965, 297-9 Chem. Abst. 66, 4983h, 1967, report the production of sodium alkyl hydroxamates by reacting the methyl ester of a C.sub.7,9 carboxylic acid with an aqueous solution of hydroxylamine sulfate and NaOH at a molar ratio of 1:1.22:2.2 and a temperature of 55.degree. C. or below.
Shchukina et. al., Khim. Prom., Moscow, 1970, 49(3) 220, report a yield of only 72 to 78 percent of the free C.sub.7,9 hydroxamic acid by reacting the methyl ester, hydroxylamine sulfate, and sodium hydroxide for two hours at 20.degree.-25.degree. C. and one hour at 55.degree.-60.degree. C., followed by acidification to pH 4-5 at temperatures below 40.degree. C. Shchukina et al., in Sin. Primen. Novykh Proverkh. Veshchestv, 1973, 123-31 reported in C.A. 80, 1974, 95199K, also report a simple lab method for the production of a reagent designated as IM-50 from C.sub.7,9 esters.
Russian workers, in Russian Patent No. 390,074, Chem. Abst. 79, 115162C (1973), and in Zh. Prikl, Khim, (Leningrad) 1972 45(8), 1895-7, Chem. Abstract 78, 29193m 1973, report improved yields with the use of 3 to 5 percent of an anionic emulsifier in an alkaline aqueous medium. The use of an anionic surfactant such as sodium lauryl sulfate (3-5 percent based on the weight of the methyl ester), reportedly gave an improved yield of 61.2 percent for valerihydroaxmic acid and 89 percent for caprihydroxamic acid. To obtain the claimed yields, however, a 40 molar percent excess of hydroxylamine hydrochloride or sulfate was required. Moreover, both the sodium salts and the free hydroxamic acids recovered are solids, which are difficult to handle and process.
Russian Patent No. 513,970, May 15, 1976, Chem. Abst. 85, 66277g, 1976, discloses the formation of a solution of mixed free C.sub.3 to C.sub.11 hydroxamic acids in hydrocarbons for use as a flotation agent. The disclosed hydroxamic acids were formed by treating carboxylate esters with the sulfate salt of hydroxylamine in an alkali medium, and then treating the resulting sodium alkyl hydroxamates with a mineral acid in the presence of 100-250 weight percent of a hydrocarbon containing less than 20 percent polar organic components, e.g., higher alcohols or esters. The aqueous layer containing NaCl or Na.sub.2 SO.sub.4 was discarded as effluent. Because of the incomplete reaction of the starting ester, this process is inefficient, producing a product that contains significant quantities of the unreacted starting ester.
U.S. Pat. No. 4,629,556 discloses the removal of various colored impurities from kaolin clays utilizing alkyl, aryl or alkyl aryl hydroxamates as collectors. The disclosed hydroxamates are produced by reacting free hydroxylamine with the methyl ester of an organic acid having an appropriate length hydrocarbon chain and configuration in a non-aqueous medium, such as methanol, in a manner similar to the methods discussed above.
U.S. Pat. No. 4,871,466 discloses a method for the production of alkyl or alkaryl hydroxamic acids and/or salts. In the disclosed method, the methyl or ethyl ester of a fatty acid having 6 to 22 carbon atoms is reacted with a hydroxylamine salt and an alkali metal hydroxide in the presence of a mixture of water, a C.sub.8 to C.sub.22 alcohol, and, preferably, a non-ionic or cationic surfactant. The disclosed reaction results in the formation of a hydroxamate solution, which can be used without further processing in the froth flotation of non-sulfide minerals, or acidified to form a liquid alcohol solution of the acid before use in the flotation process. The disclosed process eliminates the need for hazardous and expensive recovery steps, such as filtration, it is relatively rapid, taking only three to five hours for completion, and provides relatively high conversions to hydroxamates. However, the final product of the disclosed method contains some unreacted starting ester.
Improvements in the industrial production and performance of the alkyl hydroxamate collectors are still required. For example, the handling of solid products is difficult in large scale of production, and increases the complexity and cost of manufacturing. Although this problem may be overcome by carrying out the reaction in the presence of alcohols, as taught in U.S. Pat. No. 4,871,466, as discussed above, the use of C.sub.8 to C.sub.22 alcohols leads to reduced yields through the competing reaction of transesterification and hydrolysis of the methyl esters, e.g., carboxylic acids and other carbonyl components derived from the starting ester. In addition, where hydroxamic acid collectors are used in the flotation process, the shorter chain alcohols, e.g., C.sub.8, can produce uncontrollable frothing or produce undesirable froth properties, enhancing the recovery of undesirable minerals, and longer chain alcohols, i.e., C.sub.10 and above, can reduce frothing substantially, which is a serious concern in column flotation where a certain amount of controlled froth phase is necessary. Furthermore, in certain applications, depending on the value mineral being floated, the higher alcohols can adsorb on the value mineral in a reverse configuration, i.e. they can adsorb with the polar group exposed to the water phase, thereby reducing hydrophobicity on the value mineral being imparted by the alkyl hydroxamic acid, resulting in the reduced recovery of the value mineral. The commercial alcohols, which can be expensive, also have a very strong, sometimes offensive odor, which varies with chain length.
Therefore, there remains a need for alkyl hydroxamic acid collectors and a process for preparing such collectors that overcome the problems discussed above. The present invention provides such collectors and a process for preparing them.